THE ORGANIC COMPONENTS OF PLANTS 19 



groups of compounds, as they are synthetized by plants, consti- 

 tutes the major portion of the study of the chemistry of plant 

 life as presented in this book. Following the discussion of 

 the compounds themselves, the chapters dealing with enzymes, 

 with the colloidal nature of protoplasm, and with the supposed 

 accessory stimulating agencies, aim to show how the manufac- 

 turing machine known as the plant cell accomplishes its remark- 

 able results, so far as the process is now understood. 



PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE 



In connection with the discussion of each of the above-men- 

 tioned groups of organic components of plants, an attempt will be 

 made to point out what significance these particular compounds 

 have in the plant's life and growth. Certain terms will be used 

 to designate different roles, which it is probably necessary to 

 define. 



There may be two possible explanations of, or reasons for, the 

 presence of any given type of compound in the tissues of any partic- 

 ular species of plant. First, it may be supposed that this partic- 

 ular type of compounds is elaborated by the plant to satisfy its 

 own physiological needs, or for the purpose of storing it up in the 

 seeds 'as synergic food for the growth of the embryo, in order to 

 reproduce the species. For this role of the various organic food 

 materials,, etc., we will employ the term " physiological use." 

 On the other hand, it is often conceivable that certain types of 

 compounds, which have properties that make them markedly 

 attractive (or repellent) as a food for animals and men, or which 

 are strongly antiseptic in character, or which have some other 

 definite relationship to other living organisms, have had much 

 to do with the survival of the particular species which elaboratet 

 them, in the competitive struggle for existence; or have been 

 developed in the plant by the evolutionary process of " natural 

 selection." For this relation of the compound to the plant's 

 vital needs, we will use the term " biological significance." Such 

 a segregation of the roles which the different compounds play in 

 the plant's economy may be more or less arbitrary in many cases; 

 but it will be clear that when physiological uses are discussed, refer- 

 ence is being made to the plant's own internal needs; while the 

 phrase biological significance will be understood to refer to the 

 relation of the plant to other living organisms. 



